This document discusses integrating authoritative and volunteered geographic information using an ontological approach. It presents the problems caused by semantic heterogeneity between different data sources. The proposed approach uses a domain ontology and R2RML mappings to provide a common conceptualization and allow flexible integration of datasets in RDF. Current work is analyzing semantic heterogeneity in OpenStreetMap tags by studying how tags used for real-world features vary over spatial scale and time. Future work includes developing the ontology further and creating a user interface for the R2RML mappings.